Since polypropylene stretched films are inexpensive and excellent in transparency, surface gloss, heat resistance, and mechanical physical properties (for example, rigidity (so-called “nerve” of film), strength, and impact resistance), they are used for various packaging materials for a food application, an industrial application, or the like, electric materials, such as condensers or capacitors, etc., and the like, and their demands are rapidly increasing. As a general production method of such a polypropylene stretched film, there are exemplified a simultaneous biaxial stretching method by a tubular mode, a sequential biaxial stretching method (tenter mode) using a heating roll and a tenter, and the like. Furthermore, in the tenter mode, the stretching is carried out usually in a stretch ratio of a machine direction (MD: machine direction, also referred to as “flow direction”) of the film and a direction (TD: transverse direction, also referred to as “lateral direction”) perpendicular to the machine direction of 5 to 12 times. In addition, at this time, it is general that the stretching is carried out usually at a stretch speed of 200 to 525 m/min.
At present, the development of a polypropylene stretched film which is thin and has high rigidity and transparency as compared with conventional stretched films is studied. However, in polypropylene raw materials which have been conventionally used, the aforementioned production condition is substantially a maximum critical condition, and there is a concern that under a more critical condition than this, breakage of the film is generated, so that the production cannot be stably achieved. In addition, from the viewpoint of improving the production efficiency, or the viewpoint of decreasing a total cost, as compared with the conventional method, it is demanded to inhibit a breakage frequency at the time of stretching or to use high-rigidity polypropylene as a raw material. However, according to conventional polypropylene raw materials, in order to respond to the aforementioned demand, there is a limit. In addition, even if the conventional polypropylene raw materials are stretched by force, there is also a concern that the quality of a final product is deteriorated.
As for a measure for improving stretchability of the film, there is exemplified an improvement of a base resin of the film material.
For example, PTL 1 discloses that a propylene homopolymer in which a melt index and a pentad fraction fall within specified ranges, and a relation between an isotactic index and a pentad fraction and a relation between a melting point and a melting enthalpy satisfy specified formulae, respectively exhibits good stretching. However, even if the propylene homopolymer in which a very restricted primary structure is prescribed is suitable for a specified application, there was also involved such an aspect that it is poor in versatility.
PTL 2 discloses that a propylene copolymer obtained by copolymerizing propylene with a small amount of ethylene exhibits good stretching. However, there was involved such a defect that by forming a copolymer having the aforementioned constitution, the heat resistance of the copolymer or the rigidity of the stretched film is lowered.
Meanwhile, there is also proposed a measure for improving the stretchability of a film by using a second component as an additive without changing a base resin of a film material.
For example, PTL 3 discloses that a composition, in which 100 parts by weight of a polypropylene homopolymer, whose stereoregularity of a boiling-heptane insoluble part thereof is 0.960 or more in terms of an isotactic pentad fraction required by 13C-NMR, is blended with 0.01 to 2.5 parts by weight of an esterified product of dipentaerythritol as a plasticizer, exhibits good stretching. However, the esterified product of dipentaerythritol encountered such a defect that on the occasion when a molten resin thereof comes out from a die, its volatile component contaminates the surroundings of an extruder, a molded film, and the like.
In addition, PTL 4 discloses that good stretching is exhibited by adding a small amount of amorphous polypropylene to crystalline polypropylene that is a base material. However, there was involved such a defect that as the amorphous polypropylene is added, though the stretchability becomes good, the heat resistance of the resulting blended product of polypropylene is lowered, and the rigidity of a film obtained by stretching is also lowered.
In addition, PTL 5 discloses a polypropylene resin composition formed of 1 to 20 wt % of a propylene.α-olefin random copolymer (A) that is a polymer obtained through random copolymerization of propylene and an α-olefin other than propylene and having 2 to 20 carbon atoms, the copolymer having a melting point range as measured by differential scanning calorimetry (DSC) of from 40 to 115° C. and having a content of the α-olefin other than propylene of 5 to 70 mol %, and 80 to 99 wt % of a polypropylene resin (B) and discloses that this polypropylene resin composition exhibits good stretching. However, there was involved such a defect that as the propylene.α-olefin random copolymer (A) is added, though the stretchability becomes good, the heat resistance of the resulting polypropylene resin composition is lowered, and the rigidity of a film obtained by stretching is also lowered. Meanwhile, when it is contemplated to maintain the aforementioned heat resistance or rigidity of the film, conversely, there was involved such a defect that a sufficient improving effect of stretchability in the polypropylene resin composition is not obtained.